As you can see the recipient parameter is equal to ctl02 which is UniqueID of the user control. It got there when you passed this in GetPostBackEventReference call. eventArgument value is assigned to __EVENTARGUMENT hidden field and then submitted with form. This is a second parameter of GetPostBackEventReference call.

So second paratemer of GetPostBackEventReference is always encoded as string by internal class System.Web.UI.Util.QuoteJScriptString method.